Instrumentation Basics: 4 - 20mA and 3 - 15psi Control Signals ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Instrumentation Basics: 4 - 20mA and 3 - 15psi Control Signals

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In the field of instrumentation, analogue electronic signals and pneumatic signals are typically used for control purposes to actuate the final control element in a control loop which is usually a control valve.
An “analogue” electronic signal is a voltage or current whose magnitude represents some physical measurement or control quantity.

The most popular form of signal transmission used in modern industrial instrumentation systems is the 4-20 mA DC standard. This is an analog signal standard, meaning that the electric current is used to proportionately represent measurements or control signals. Typically, a 4 mA current value represents 0% of scale, a 20 mA current value represents 100% of scale, and any current value in between 4 and 20 mA represents a commensurate percentage in between 0% and 100%.

In pneumatic systems, a standard signal range of 3 to 15 PSI (Pounds Per Square Inch) is used. Here, a varying air pressure signal represents some process measurement in an analogue (proportional) fashion. Typically, a 3 PSI pressure value represents 0% of scale, a 15 PSI pressure value represents 100% of scale, and any pressure value in between 3 and 15 PSI represents a commensurate percentage in between 0% and 100%. It is worthy of note to state here that pneumatic signals are commonly used in process industries for safety especially when there is a risk of fire or explosion


Relating 4-20 mA signals to instrument variables:
Calculating the equivalent milliamp value for any given percentage of signal range is quite easy. Given the linear relationship between signal percentage and milliamps, the equation takes the form of the standard slope-intercept line equation C = mP + b.
Here, C is the equivalent current in milliamps, P is the desired percentage of signal, m is the span of the 4-20 mA range (16 mA), and b is the offset value, or 4 mA:
Current = (16mA)(P/100%) + (4mA), P = percentage range of signal

This equation form is identical to the one used to calculate pneumatic instrument signal pressures (the 3 to 15 PSI standard):

Pressure = (12 PSI)(P/100%) + (3 PSI)

The same mathematical relationship holds for any linear measurement range. Given a percentage of range P, the measured variable is equal to:

Measured variable = (Span)(P/100%) + (LRV)
 
Practical examples of calculations between milliamp current values and process variable values follow:
(A) An electronic temperature transmitter is ranged 40 to 140 degrees Fahrenheit and has a 4-20 mA
output signal. Calculate the current output by this transmitter if the measured temperature is 60 degrees Fahrenheit.

Solution:
First, we convert the temperature value of 60 degrees into a percentage of range based on the knowledge of the temperature range span (140 degrees − 40 degrees = 100 degrees) and lower-range value (LRV = 40 degrees).
We may do so by manipulating the general formula:


Measured variable = (Span)(P/100%) + (LRV)
Measured variable – (LRV) = (Span)(P/100%)

Therefore, P = [(Measured variable – LRV)/(Span)] x 100%
= [(60 – 40)/(100)] x 100% = 20%

Next, we take this percentage value and translate it into a 4-20 mA current value using the formula:

Current = (16mA)(P/100%) + (4mA)
= (16mA)(20%/100%) + (4mA) = 7.2 mA

Therefore, the transmitter should output a Process Value signal of 7.2mA at a temperature of 60 F.

(B) An electronic loop controller outputs a signal of 8mA to a direct-responding control valve (where 4 mA is shut and 20 mA is wide open). How far open should the control valve be at this Manipulated Variable signal level?

Solution:
We must convert the milliamp signal value into a percentage of valve travel. This means determining the percentage value of the 8mA signal on the 4-20 mA range. First, we need to manipulate the percentage-milliamp formula to solve for percentage (P):

Current = (16mA)(P/100%) + (4mA)

P/100% = [(Current – 4mA )/(16mA)]

P = [(Current – 4mA )/(16mA)] x 100%

Next, we plug in the 8mA signal value and solve for P:

Therefore, P = [(8mA – 4mA )/(16mA)] x 100% = 25%

Therefore, the control valve should be 25 % open when the MV signal is at a value of 8mA.

Additional learning resources:
Now that you understand basic control signals used in instrumentation. You could also check out this book: Measurement and Control Basics It is a very good book for beginners to instrumentation and control. Established technicians and  engineers could also get the book for their library.

Closely related to the concept of the 4-20mA signal and 3-15psi signal is the art of calibration. Many technicians and experts in instrumentation and control will agree with me that calibration is a critical aspect of plant management and process safety.
To learn the art of calibration you might check out these books:
A Technician's Guide to Calibration
Quality Calibration Handbook